This invention relates to the art of radar systems, and more particularly to a new and improved short pulse high resolution radar system and method for subsurface inspection.
One area of use of the present invention is subsurface inspection of non-metallic building materials such as pavements of highways and bridge decks and similar roadway surfaces, although the principles of the present invention can be variously applied. A downward looking, short pulse high resolution radar for subsurface inspection is different from conventional radars in that it generally has a limited range capability, on the order of several feet as compared to several hundred miles for a conventional radar. The term subsurface is intended to include the region immediately adjacent and below the exposed, visible, outside or external surface of the pavement or other medium being inspected. Also, the free space resolution in the short pulse system is on the order of a few inches, i.e. it can separate in distance two targets a few inches apart, whereas the conventional system can separate in distance two objects no less than several hundred feet apart. The inherent resolution of any radar system is directly related to the duration of the transmitted pulse or its bandwidth. The shorter the pulse or wider the bandwidth the higher the resolution of the system. For example, a one nanosecond pulse has a resolution of 5.9 inches in free space or about 2.25 inches in concrete. For a ground probing, high resolution radar the design represents a compromise between wide bandwidth and lower center frequency. The lower the center frequency the greater the depth of penetration but the more difficult it is to achieve a wide bandwidth or high resolution system.
The typical highway inspection radar includes a short pulse transmitter, typically generating one cycle of RF and on the order of one nanosecond in duration. This signal is radiated by a downward looking, wide band, non-dispersive antenna which produces minimal transmitter signal distortion. The received signals or echoes from the target are coupled by the antenna into the sampling receiver. A sampling receiver converts a short pulse, very wide band signal to a narrow bandwidth signal. This process is identical to that employed by commercially available sampling oscilloscopes. The first external signal detected is the reflection from the surface of the pavement followed by reflections generated by disturbances from within the pavement, such as delamination or deterioration, and then followed by signals from the bottom of the pavement or below the pavement such as voids.
In addition to external signals detected by the receiver, internal multiple reflections generated by non-ideal system components such as the antenna, receiver, transmitter, transmission lines and RF connectors, also are detected. These internal multiple reflections or internal clutter interfere with the desired external echoes from within the pavement. This makes detection and classification of pavement disturbances difficult and also makes evaluation of the internal condition of the highway pavement less reliable. This source of clutter is the principle factor limiting system performance of inspection radars heretofore available. Thermal noise is not a significant problem with this type of very short range radar as it is with conventional radars. Therefore, a reduction in internal clutter alone would significantly enhance the system detection capability and reliability of evaluation in highway and bridge deck inspection. Earlier ground penetrating radars did not recognize the problem of eliminating internally generated clutter. The radar system and method of the present invention addresses this problem.
Another problem associated with earlier ground penetrating radars, especially non-ground-contacting radars, is the undesirable operation of the sampling receiver process which utilizes the first external echo arising from the ground surface to control the start of the sampling process. This problem arises due to the wide variation in amplitude of the surface echo because of wide variation in the reflective properties of the ground media which can include pavement, soil, sand, gravel and metal and rubber inserts such as expansion joints. The radar system and method also addresses this problem.